Wave soldering is a common method of forming solder joints between electronic components and circuit traces on a printed circuit board. Electronic components are placed on a circuit board and their leads are inserted into holes in the circuit board so that the leads touch the metal pads to which they are to be soldered. The components may be glued to the circuit board to retain them during the soldering process.
With the components in place, an applicator applies flux to the bottom of the board in the form of a spray, a foam or a wave. Flux allows soldering of metallic materials with poor wetability and solderability, such as oxidized copper. Flux also allows solder to fill metallized holes in the board more readily.
The fluxed board is preheated to dry and activate the flux and to thermally prepare the board to contact the molten solder with low thermal stress. The activated flux reacts with metal oxides on the component leads and the circuit board pads and dissolves the oxides. The presence of oxygen as in air has been thought to be deleterious in the preheat operation since oxygen produces metal oxides.
The bottom of the fluxed, preheated board is contacted with molten solder either in a static bath or in a pumped wave so as to wet the parts to be coated or joined with solder. Upon detachment of the board from the solder bath, a coating of solder remains on the wetted parts. The adhering solder solidifies, forming electrically conductive joints and coatings.
After soldering, the board is usually cleaned to remove the remaining flux and flux residues, which can cause corrosion, unwanted electrical conduction, poor appearance and interference with subsequent testing. Cleaning, however, is desirably eliminated since it is expensive and cleaning fluids are environmentally objectionable. Subsequent inspection or testing determines what connections, desired and undesired, have been made by the solder. The testing is usually performed by an array of pins brought into contact with the board pads and through which electrical measurements are made.
Most of the flux applied to a board remains on the board after the solder contact. Thus, if the flux layer is thick, a test pin may not penetrate to establish conductive contact with an intended test point on the board, and a false open will be indicated. A long delay between soldering and inspection will allow flux to harden, and, if not removed, will particularly impede pin penetration.
Several types of soldering defects occur most frequently. A common defect is an incomplete or missing solder deposit where joining was intended, thereby causing an open. Another is bridging of solder between metalized portions on the board where joining was unintended, thereby causing a short. Still another is failure of the solder to fill a metallized hole in the board.
To eliminate post-soldering cleaning and false pin testing results, no-clean fluxes and special flux application techniques have been developed. A no-clean flux is a flux that after solder contact leaves a low level of residue which is noncorrosive and nonconductive. Preferably a no-clean flux contains little or no halide, but most preferably a non-corrosive, non-conductive organic acid dissolved in a solvent such as ethanol or isopropanol. Common RMA flux is a no-clean flux consisting of a mixture of rosin (abietic acid), activator (dimethylamine hydrochloride) and solvent (alcohol). Another no-clean flux is adipic acid (1% by weight) in ethyl or isopropyl alcohol. To avoid false pin test results, known as contact defects, no-clean flux desirably is applied in a thin layer. The following table shows the relationship between the thickness of an RMA flux layer and observed air atmosphere soldering and test contact defects. The data are from Soldering In Electronics by Wassink and Klein, 1984, page 235.
______________________________________ Flux Contact Soldering Thickness, Defects, Defects, microns per million joints Type ______________________________________ 15 3,333 4 333 bridging 2 50 bridging, poor hole filling ______________________________________
The results indicate that as flux thickness is reduced, test contact defects decrease and soldering defects increase.
Conventional wave soldering machines are available which apply flux to a circuit board, preheat the board, contact the board with a molten wave of solder and then detach the board from the solder wave. The board is transported sequentially by a conveyer through these operations, which are performed in air. Typically the machine configuration comprises a fluxer, preheater and solder pot and conveyer mounted on a frame and enclosed by a liftable cover on a hinge. A slight vacuum is applied at a port at a central point in the cover to draw off objectional fumes emanating from these operations. Electrical controls which may be governed by a microprocessor are provided for various adjustments. More recently, wave soldering machines have been designed to flux, preheat and solder circuit boards in an inert or protective atmosphere. These machines provide benefits over machines which perform these operations in air as follows:
1. large reduction in the amount of solder oxides (dross) formed on the molten solder surfaces;
2. improved wetting of the solder on metal surfaces on a circuit board;
3. improved wicking of the solder into holes and through holes in the circuit board;
4. reduced open defects;
5. elimination of solder icicle formation;
6. capability of solderinq more closely spaced components and pads with acceptable bridging defect rates;
7. reduced amount of flux required;
8. reduced soldering machine cleaning and maintenance requirements; and
9. elimination of board cleaning after soldering, providing a minimal layer of no-clean flux was applied.
A protective atmosphere under which wave soldering is performed with the benefits mentioned comprises a non-oxidizing gas and not more than 5 percent oxygen, preferably not more than 100 ppm oxygen, and most preferably not more than 10 ppm oxygen. Nitrogen is a satisfactory non-oxidizing gas in which to perform the contacting with solder, and because of its low cost, nitrogen is a preferred non-oxidizing gas.
To achieve and maintain the protective atmosphere, the various operations are conducted in a long continuous enclosure or series of joined tunnels. Typical apparatus is described in U.S. Pat. No. 4,921,156 to Hohnerlein. The protective atmosphere is introduced into the tunnel enclosing the solder pot and flows out through the work entrance tunnel and the work egress tunnel. To restrict the escape of protective atmosphere, seal flaps are provided in the tunnels. The flaps are tilted open in the transport direction by a passing workpiece and close thereafter. Thus Hohnerlein's fluxing, preheating, solder attachment, detachment, and cooling are under a protective atmosphere.
Alternatively, gas jets have been used to form gas curtains and provide gas flow barriers at specific locations in tunnels as described in U.S. Pat. No. 4,538,757 to Bertiger.
Still another technique, described by Schouten in Circuits Manufacturing, September 1989 pages 51-53, has been to provide chambers in the entrance tunnel and the egress tunnel. Boards pass intermittently through the chambers which open and close. Within a chamber, when closed, a vacuum is drawn. The chamber is then filled and flushed with a protective atmosphere. This process is repeated allowing the oxygen content in the soldering zone to be kept below 10 ppm. The protective atmosphere used is nitrogen.
Many wave soldering machines designed for use in air are in operation in industry. Despite the benefits of soldering under a protective atmosphere, it is difficult for a circuit board manufacturer to justify the replacement of an existing machine designed to solder in air with a new machine designed to solder under a protective atmosphere. The operating savings that might be realized would take several years to off-set the cost of the new machine.
An alternative to a new soldering machine designed to solder in a protective atmosphere is to retrofit an existing machine so that it can be operated in a protective atmosphere Heretofore, wave soldering machines initially designed to solder printed circuit boards under a protective atmosphere have provided a protective atmosphere for all the functions of fluxing, preheating, contacting with solder, separation from solder and cooling of the board. It has been believed that a protective atmosphere for all these functions was necessary to achieve the benefits of soldering in a protective atmosphere as enumerated earlier. However, to provide a protective atmosphere for all these sections of a conventional air soldering machine is significant in terms of cost, additional complexity and retrofit time.
Thus there is a need for an apparatus for retrofitting an air soldering machine which minimizes the amount of additional installation required. A method and apparatus requiring a protective atmosphere only over the soldering portion of the machine itself would be very attractive.
It is an object of this invention to provide a method and apparatus whereby existing wave soldering machines originally designed to operate in air are retrofitted to obtain the benefits of soldering machines designed to operate under a protective atmosphere.
It is also an object of this invention to provide an economical design for new wave soldering machines initially intended to operate under a protective atmosphere.
It is a feature of this invention that only the solder pot and the immediate space over the solder pot need be provided with a protective atmosphere, allowing fluxing, preheating and cooling to be performed in air.
It is another feature of this invention that the protective atmosphere used may contain up to 5% oxygen in the solder contacting region.
It is an advantage of this invention that the retrofit of wave soldering machines designed to operate in air is economical and speedy to accomplish.
It is a further advantage that low soldering defect rates are achieved with reduced usage of flux which eliminates the need for cleaning of circuit boards after soldering.
It is another advantage of this invention that the protective atmosphere may be generated by separation of air by membrane or pressure swing adsorption, or by partial combustion of air.